Synthesis, photoluminescent features and intramolecular energy transfer mechanism of europium (III) complexes with fluorinate β-diketone ligand and auxiliary ligands

Article abstract of DOI:10.1016/j.jfluchem.2015.06.011

Abstract A series of novel luminescent europium (III) complexes based on fluorinated β-diketone ligand 4,4-difluoro-1-phenyl-1,3-butanedione (DPBD) and auxiliary ligands 2,2-biquinoline (biq) or 1,10-phenanthroline (phen) or neocuproine (neo) or 2,2-bipyr

Full text of DOI:10.1016/j.jfluchem.2015.06.011

Journal of Fluorine Chemistry 178 (2015) 6–13
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Synthesis, photoluminescent features and intramolecular energy
transfer mechanism of europium (III) complexes with fluorinate
b
-diketone ligand and auxiliary ligands
*
Manju Bala, Satish Kumar, V.B. Taxak, Priti Boora, S.P. Khatkar
Department of Chemistry, Maharshi Dayanand University, Rohtak 124001, Haryana, India
A R T I C L E I N F O
A B S T R A C T
A series of novel luminescent europium (III) complexes based on fluorinated
Article history:
b
-diketone ligand 4,4-
Received 19 December 2014
Received in revised form 10 April 2015
Accepted 12 June 2015
difluoro-1-phenyl-1,3-butanedione (DPBD) and auxiliary ligands 2,2-biquinoline (biq) or 1,10-
phenanthroline (phen) or neocuproine (neo) or 2,2-bipyridyl (bipy) have been synthesized. The ligand
DPBD with their complexes Eu(DPBD)3ꢀ(H2O)2 (C1), Eu(DPBD)3ꢀbiq (C2), Eu(DPBD)3ꢀphen (C3),
Eu(DPBD)3ꢀneo (C4) and Eu(DPBD)3ꢀbipy (C5) were confirmed by elemental analysis, infrared (IR)
and proton nuclear magnetic resonance (NMR) spectroscopy. The crystalline nature, photoluminescence
and thermal behavior were investigated by powder X-ray diffraction (XRD), photoluminescence (PL)
spectroscopy and TG/DTA-DSC respectively. The emission spectra of complexes showed the
characteristics sharp bands in solid state corresponding to 5D0–7Fj (j = 0–3) transition of europium
ion with 5D0–7F2 as the most intense transition. The emission spectra, energy transfer mechanism,
luminescence decay time and relative quantum efficiency clearly reveals that these photophysical
Available online 23 June 2015
Keywords:
4,4-Difluoro-1-phenyl-1,3-butanedione
Photoluminescent
Europium(III)
IR
NMR
properties are greatly influenced by the p-conjugated system, stability of the complexes and the efficient
energy transfer from DPBD ligand to emitting level of Eu3+ ion. The complex Eu(DPBD)3ꢀbipy (C5)
exhibited highest quantum efficiency, luminescence intensity bearing good CIE color coordinates
(x = 0.64 and y = 0.34) matching the NTSC (National Television Standard Committee) standard values for
the pure red color and longest lifetime which makes it a promising red-emitting component for display
devices.
ß 2015 Elsevier B.V. All rights reserved.
1. Introduction
absorption by the ligand, rate of energy transfer from ligand to
central metal ion, radiative emission rates and non-radiative decays.
The brightly electroluminescent europium complexes reported
by Weismann (1942) and its devices by Kido (1991) opened new
vistas in the field of luminescence [1,2] since then the luminescence
properties of europium complexes have been intensively investi-
gated. These europium complexes established a group of materials
withenormous potential application in material science like sensory
material [3], luminescent materials [4], light emitting diodes [5],
The 4f-4f intra-configurational forbidden transition leads to feeble
intensity of absorption and emission spectra. In order to overcome
this disadvantage, a chelating organic chromophore with magnifi-
cent absorption coefficient is employed. So,
b-diketone with
extended -conjugation system fulfill above necessity making
p
them highly luminescent materials. In electroluminescent com-
plexes the efficient energy transfer and high absorption coefficient
can be accomplished by selection of appropriate ionizable organic
chelating moiety which can coordinate to metal ion constituting an
eight, nine or twelve coordinated neutral complex. Among 8–12
possiblecoordinationsitesofcentralmetalion, six coordinativesites
laser materials and optical fiber [6]. The scrutiny of
b-diketonato
based europium complexes has surpassed the other europium
complexes in last decades owing to the intriguing luminescent
features such as high efficiency with long fluorescence life time and
sharp emission bands in visible region having FWHM (Full Width at
Half Maxima) in the range of 4–10 nm [7–9]. The 4f-4f photo-
luminescence intensity is the consequence of a harmony between
are engaged with the three bidentate
b-diketones in coordination
sphere. Rest of coordination environment is occupied by water or
solvent molecules or second auxiliary ligand. But C–H, O–H and N–H
stretching vibration in complexes decrease the photoluminescence
intensity by vibronic quenching [10,11] via back energy transfer to
theligand-localizedelectronicstate[12]orbytheexchangefromthe
4fn configuration to the LMCT (ligand to metal charge transfer) [13].
*
Corresponding author. Tel.: +91 9813805666.
E-mail address: s_khatkar@rediffmail.com (S.P. Khatkar).
http://dx.doi.org/10.1016/j.jfluchem.2015.06.011
0022-1139/ß 2015 Elsevier B.V. All rights reserved.

M. Bala et al. / Journal of Fluorine Chemistry 178 (2015) 6–13
7
Table 1
The elemental analytical data of europium (III) complexes C1–C5.
Complexes
C (%) found
(cal.)
H (%) found
(cal.)
N (%) found
(cal.)
Eu (%) found
(cal.)
DPBD
C1
59.96 (60.60)
45.96 (46.21)
57.46 (57.65)
54.40 (54.60)
55.27 (55.52)
53.21 (53.39)
3.92 (4.04)
2.92 (3.20)
2.98 (3.30)
3.04 (3.14)
3.32 (3.47)
3.06 (3.22)
–
–
–
19.02 (19.51)
14.90 (15.21)
16.12 (16.46)
15.85 (15.98)
16.78 (16.90)
C2
2.47 (2.80)
2.90 (3.03)
2.78 (2.94)
2.98 (3.11)
C3
C4
C5
The loss of energy from the C–H bonds vibration decrease the
emission intensity so the substitution of C–H bonds in a -diketone
b
moiety with lower energy C–F oscillators could decrease the back
energy transfer loss and improve the emission intensity of central
metal ion. It is well known that the spacing between the vibrational
level of C–F oscillators lower the vibration energy of organic ligand
and decrease the energy loss caused by vibration of organic ligand.
The fluorine substituents on the ligand enhance the spin-orbit
coupling via heavy atom effect confirmingly promoting the
intersystem crossing (ISC) [14–17]. To achieve the above objective,
Fig. 1. The TG/DTA-DSC curves of the complex C5.
C55C in spectra of complexes C1–C5 are red shifted 9–27 cmꢁ1
and 11–40 cmꢁ1 respectively in comparison to the free ligand
a new
b-diketonate ligand DPBD was synthesized by an ecofriendly
microwave method which embodied with fluorinated alkyl group as
well as conjugated phenyl group employing as ‘antenna’ for
europium ion to effectively photosensitize its emission [18].
In the present report, with the purpose of growing novel
photoluminescent materials, a series of europium (III) binary and
ternary complexes by using fluorinated ligand and auxiliary ligand
like 2,2-bipyridyl, 2,2-biquinoline, neocuproine and 1,10-phenan-
throline were synthesized. We have also explicated the photo-
luminescence governing study such as excitation spectra, emission
spectra, UV–vis absorption spectra, decay time of emitting metal
levels and CIE color coordinates in detail. The meticulous study of
sensitization process in these luminescent complexes elucidated
through proposed energy transfer mechanism.
DPBD ensuing the extended p-conjugated system in complexes
[20]. The absorption bands at 1571–1560 cmꢁ1 in the complex
spectra are attributed to C55N stretching vibration of auxiliary
ligands having coordination through nitrogen which is not
reflected in free ligand DPBD spectra. There are no marked
changes in C–F stretching vibration at 1270–1269 cmꢁ1 in the
spectra of complexes and ligand, indicating that it is not involved in
coordination. Two new absorption bands appeared in the
complexes spectra only at 536–522 cmꢁ1 and 428 cmꢁ1 assigned
to the stretching vibration of Eu–N and Eu–O respectively. The
above IR study clearly reveals that the ligand DPBD and auxiliary
ligands are coordinated to the central europium (III) ion via C55O,
enolic –OH of ligand and N–N groups of auxiliary ligands.
In 1H NMR, two peaks of ligand spectra show obvious changes
relative to spectra of complexes due to paramagnetism of
europium (III) ion. The proton signal of enolic –OH at 15.01 ppm
and keto –CH2 protons peak at 4.22 ppm are missing from
complexes spectra indicating that the enolic –OH involved in
coordination. The enolic –CH proton of ligand exhibits a singlet at
6.35 ppm which is shifted to upfield at 3.20–2.80 ppm in
complexes, signifying the paramagnetism of lanthanide ion [21].
The multiplet of benzene protons in ligand is moved toward higher
field in the spectra of complexes.
2. Results and discussion
2.1. Composition and spectroscopic studies of the complexes
The elemental analytical data of Carbon, Hydrogen, Nitrogen of
ligand DPBD, their synthesized complexes and Eu(III) content in
complexes C1–C5 are tabulated in Table 1. The results show that
the calculated and experimental values are close to each other,
indicating that the composition of the complexes is consistent to
Eu(DPBD)3ꢀH2O (C1), Eu(DPBD)3ꢀbiq (C2), Eu(DPBD)3ꢀphen (C3),
Eu(DPBD)3ꢀneo (C4) and Eu(DPBD)3ꢀbipy (C5).
2.2. Thermal behavior and powder X-ray study
The significant IR frequencies of ligand DPBD and complexes
C1–C5 are summarized in Table 2. Some distinct changes in ligand
spectra are observed in comparison to spectra of complexes. The
broad bands at 3430 cmꢁ1 and 3421 cmꢁ1 are detected which is
assigned to the stretching vibration of enolic –OH in ligand DPBD
and water molecules present in coordination sphere of the C1
complex respectively. The strong absorption bands of C55O and
The thermal behavior of europium complexes are perceived by
TG/DTA-DSC curves which is analyzed at a heating rate of 30 8C
minꢁ1 under nitrogen environment. The complexes C1–C5 show a
similar pattern of decomposition temperature and mass loss
percentage, therefore complex C5 is depicted as representative of
other complexes in Fig. 1. The TGA curves show three important
Table 2
The IR characteristics bands (cmꢁ1) of ligand and its europium complexes.
Complexes
n
(O–H)
n
(C5O)
n
(C5N)
n
(C5C)
n
(C–F)
n
(Eu–N)
n
(Eu–O)
–
DPBD
C1
3430 (b)
1635 (s)
1626 (s)
1622 (s)
1625 (s)
1608 (s)
1620 (s)
–
1542 (s)
1529 (s)
1525 (s)
1531 (s)
1502 (s)
1517 (s)
1270 (s)
1269 (s)
1269 (s)
1271 (s)
1269 (s)
1270 (s)
–
–
3421 (b)
428 (m)
428 (m)
428 (w)
428 (m)
428 (m)
C2
–
–
–
–
1562 (s)
1560 (s)
1571 (s)
1565 (s)
522 (s)
530 (m)
536 (s)
532 (s)
C3
C4
C5
b =broad, s = strong, m =medium, w = weak.

8
M. Bala et al. / Journal of Fluorine Chemistry 178 (2015) 6–13
consecutive mass loss steps in the 0–1100 8C temperature range.
The first mass loss observed at 202.8 8C due to loss of moisture
present in complex. The second mass loss step appear from
202.8 8C to 309.8 8C with the 82.7% weight loss due to expelling of
three DPBD and one 2,20-bipyridyl molecules from the coordina-
tion sphere of complex. At last step, the product of previous step
may be oxide of europium, which dissociate completely up to
1000 8C. In DSC curve, an endothermic peak is observed at 194 8C
where no weight loss observed, corresponds to melting point of the
complex. In addition, there are two exothermic peaks at 232 8C and
511 8C in DSC attributed to the release of three ligands with one
auxiliary ligand and decomposition of the europium oxides
respectively. In TGA curve, two exothermic peaks at 232 8C and
280 8C indicate the removal of three ligand and one 2,20-bipyridyl
molecules respectively. The results of TG/DTA-DSC analysis
suggested that the all europium complexes exhibited good thermal
stability.
In order to know the crystalline nature of ligands and
complexes, the powder XRD pattern of ligand as well as complexes
C1–C5 are investigated as depicted in Fig. 2. The characteristics
Fig. 3. The UV–vis absorption spectra of europium metal ion, 1,10-phenanthroline,
ligand DPBD and C1–C5 complexes in ethanol solution.
peaks of C2–C5 appeared in the range of 10–808 at 2u whereas a
the coordination of europium ion doesn’t affect the energy of
singlet excited state of ligand DPBD significantly [22]. The
europium metal ion and 1,10-phenanthroline exhibit absorption
at lower range 41666.67–37037.04 cmꢁ1 which is assigned to the
regular pattern was obtained for C1, clearly indicating the
crystalline nature of C2–C5 and amorphous nature of C1, which
may be the results of water molecules present in coordination
sphere of C1. The diffraction angles, diffraction intensity in XRD
pattern of the complexes are different from those of ligand DPBD,
bipy and phen which infer the formation of new crystalline
complexes. The particle size can be estimated with the help of
p–p* transitions of ligand only. In addition, 1,10-phenanthroline
shows absorption at 38,461.54 cmꢁ1, suggesting that the forma-
tion of Eu–N affects the absorption noticeably shifting it toward
higher wavelength in complexes as a consequence of extended
conjugation.
Scherrer’s equation D = 0.941
particle size, the X-ray wavelength (0.15406 nm),
angle and is full width at half maxima (FWHM in radian).
l
/b
cos
u
where D is the average
l
u
is diffraction
b
2.4. Photoluminescent features
Therefore the particle sizes of C2–C5 are 64.5 nm, 63.9 nm,
67.8 nm and 54.8 nm respectively. The above powder X-ray
diffraction study confirms the formation of nano-crystalline
complexes.
The excitation spectra of ligand DPBD as well as their complexes
C1–C5 by monitoring 5D0–7F2 transition ( em = 16,313.21 cmꢁ1) is
l
displayed in Fig. 4. All the spectra of complexes exhibit a broad
band from 40,000.00 cmꢁ1 to 22,222.22 cmꢁ1 and a weak band at
21,551.72 cmꢁ1 ascribed to the
p–p* transition of coordinated
2.3. UV–vis analysis
ligand DPBD and absorption of central europium ion respectively.
The red shifting of ligand maxima (28,901.73 cmꢁ1) is noticed in
complexes (27,027.03–26,525.20 cmꢁ1) which explain the effi-
Fig. 3 illustrates the UV–vis absorption spectra of ligand DPBD,
one of auxiliary ligand 1,10-phenanthroline, europium metal ion
and their complexes C1–C5 in ethanol solution (10ꢁ5 mol/L) at
room temperature. The absorption maxima of free ligand
cient sensitization phenomenon between
b-diketone ligand and
metal ion in which the ligand DPBD act as an ‘antenna’.
(35,460.99 cmꢁ1
)
and that of the complexes (35714.29–
The emission spectra of complexes C1–C5 are recorded at
35460.99 cmꢁ1) approach to nearly same value, indicating that
l
ex = 26,881.72–26,525.20 cmꢁ1 with 400 PMT, depicted in Fig. 5.
Fig. 2. The powder XRD pattern of ligand DPBD, 2,20-bipyridyl, 1,10-phenanthroline
and C1–C5 complexes at room temperature.
Fig. 4. The excitation spectra of ligand DPBD and their europium complexes C1–C5
in solid state at room temperature monitored at 613 nm.

M. Bala et al. / Journal of Fluorine Chemistry 178 (2015) 6–13
9
[26–28]. The luminescent intensity ratio (I1/I2 = 5D0–7F2/5D0–7F1)
of electric to magnetic dipole transitions show the symmetry of
the coordination sphere as well as monochromaticity properties
of metal complexes, their values are found to be in the range 7.0–
10.73, being the highest for complex C5. Fig. 5 clearly shows that
the luminescence intensity of complexes C2–C5 possessing
auxiliary ligands is greater than complex Eu(DPBD)3ꢀ(H2O)2
(C1) owning to the quenched O–H vibrations in coordination
sphere, which is consequence of stability and extended
p-
conjugation of auxiliary ligands in C2–C5 complexes which lower
the energy level of ligands, constituting the efficient way of energy
transfer from ligand to the emitting level of metal ion [29]. The
order of emission intensity expounded by the nature of auxiliary
ligands, the ligands with same framework i.e. bipyridyl and
biquinoline possessing steric effect, makes the biquinoline
complex lowest and bipyridyl complex highest intense among
the ternary complexes. On the other hand, among 1,10-
phenanthroline and neocuproine auxiliary ligands which have
the similar framework, the electron releasing effect of two methyl
groups in neocuproine makes the complex to emit at higher
intensity than phenanthroline complex. In addition, these
auxiliary ligands have high affinity toward lanthanide ions [30],
therefore their company anticipates 4f-4f forbidden transitions by
inducing an asymmetric coordination environment around metal
ion which increase the intensity of the complexes.
Fig. 5. The emission spectra of C1–C5 complexes at room temperature in solid state.
Fig. 6 depicts the emission color of the complexes C1–C5
calculated from their PL spectra and assured by Commission
Internationale de Eclairage (CIE) diagram, enlisted in Table 3. The
color coordinates of the all complexes C1, x = 0.5018 and y = 0.2742;
C2, x = 0.5234 and y = 0.2886; C3, x = 0.5668 and y = 0.3128; C4,
x = 0.6173 and y = 0.3304 and C5, x = 0.6465, y = 0.3424 dropped in
red region, with a spectacular shift toward pure red color with
increasing intensity. The color coordinates are close to the standard
red color of NTSC (x = 0.67, y = 0.33), SMPTE (x = 0.63, y = 0.34) and
EBU (x = 0.64, y = 0.33) as a consequence of higher red/orange ratio.
The above results of CIE indicate that complexes C1–C5 are
promising candidate of red component in OLEDs.
The luminescence decay profile of 5D0 level of metal ion
corresponding to C1–C5 complexes is investigated by monitoring
the emission at 613 nm (Fig. 7). The decay time values of C1–C5 are
calculated by the FL solution software of F-7000 spectrometer,
tabulated in Table 3. The decay curves for these complexes obey a
The pertinent data of photoluminescence are arranged in Table 3.
The spectra comprise of characteristics narrow emission at
17,241.38 cmꢁ1
,
16,920.47 cmꢁ1 and 16,313.21 cmꢁ1 due to
5D0–7F0, 5D0–7F1, and 5D0–7F2 transition of europium ion. The
low intensity emission peak at 17,241.38 cmꢁ1 due to 5D0–7F0
transition being a one-order perturbation is forbidden in electric as
well as magnetic dipole transition in accordance to theory of
Judd-Ofelt [23–25]. The emission peak of slightly higher intensity
at 16,920.47 cmꢁ1 due to 5D0–7F1 transition is magnetic dipole
transition being independent on local chemical environment can
be employed as an internal standard to infer the differences in
ligands [25–27]. The most intense peak at 16,313.21 cmꢁ1 due to
5D0–7F2 transition being electric dipole transition depends on
highly polarizable coordination environment around the europi-
um ion possessing very low value of full width at half maxima
(FWHM) i.e. 6.9 nm, 6.5 nm, 5.6 nm, 3.9 nm and 3.6 nm corre-
sponding to the complexes C1–C5, leading to intense pure red
emission. The excellent intensity of 5D0–7F2 transition as
compared to 5D0–7F1 transition points to the lack of an inversion
symmetry in the coordination environment of europium ion
single exponential decay law which can be represented as
t
I = I0 exp (ꢁt/ ) where
t
is the radiative decay time, I and I0 are
and respectively,
the luminescence intensities at time
t
0
suggesting that only single chemical environment around the
europium(III) [31]. These results show that the complexes C2–C5
having auxiliary ligands present longer lifetime relative to C1
Table 3
The luminescence data of complexes C1–C5.
Complexes
lex (nm) in cmꢁ1
lem (nm) in cmꢁ1
Transition assignments
x and y color coordinates
t (ms)
C1
26,525.20 (377)
17,241.38 (580)
16,920.47 (591)
16,313.21 (613)
17,241.38 (580)
16,920.47 (591)
16,313.21 (613)
17,241.38 (580)
16,920.47 (591)
16,313.21 (613)
17,241.38 (580)
16,920.47 (591)
16,313.21 (613)
17,241.38 (580)
16,920.47 (591)
16,313.21 (613)
5D0–7F0
5D0–7F1
5D0–7F2
5D0–7F0
5D0–7F1
5D0–7F2
5D0–7F0
5D0–7F1
5D0–7F2
5D0–7F0
5D0–7F1
5D0–7F2
5D0–7F0
5D0–7F1
5D0–7F2
0.5018, 0.2742
0.22
0.25
0.28
0.33
0.39
C2
C3
C4
C5
27,027.03 (370)
27,027.03 (370)
26,525.20 (377)
26,881.72 (372)
0.5234, 0.2886
0.5668, 0.3128
0.6173, 0.3304
0.6465, 0.3424

10
M. Bala et al. / Journal of Fluorine Chemistry 178 (2015) 6–13
Fig. 8. The proposed energy transfer mechanism in Eu(DPBD)3.phen (C3).
reference, n and nstd represent the refractive index of solvent for
complex and reference respectively. The relative quantum
efficiency is highest for C5 which may be the effect of energy
transfer by the ligand and auxiliary ligand bipy.
Fig. 6. CIE coordinates of the europium complexes C1–C5.
In order to elucidate the energy transfer mechanism in
photoluminescence process of the complexes, the proposed
mechanism of energy transfer in C3 and C5 complexes of two
different frameworks among auxiliary ligands are shown in Fig. 8
and Fig. S1 respectively. In the ligand DPBD, the energy of lowest
excited singlet state and triplet state are estimated from the edge
wavelength of UV–vis absorption spectra (Fig. 3) and the lower
emission edge wavelength of phosphorescence spectra of C6
complex (Fig. S2) respectively [33,34] because the triplet energy
level of ligand DPBD (32,154 cmꢁ1) approximately equals to lowest
excited state (6P7/2) of Gd3+ ion so the absorbed energy of ligand
can’t be transferred to gadolinium ion, hence the triplet state of the
ligand is obtained from the phosphorescence spectra. The singlet
and triplet excited states of phen as well as bipy are obtained from
literature [35]. The respective S1 and T1 values of DPBD ligand,
auxiliary ligands phen and bipy are listed in Table 4.
Incomplexes, theefficiencyofenergytransfermainlydependson
twomechanisms;in first the energy transfer takesplacefrom lowest
excited triplet level of DPBD ligand to resonance energy level of
metal ion as proposed by Dexter’s electron exchange interaction
theory [36]. According to this theory, more suitable the energy gap
between triplet level of ligand and emitting level of europium ion,
more efficient is the energy transfer influencing the luminescence
intensity. The transfer probability constant (Ps) is given as
Fig. 7. The luminescence decay profile of C1–C5 complexes monitoring at 613 nm in
powder form.
because auxiliary ligands enhance the luminescence stability of
the complexes.
!
Z
2p
Z2
h
Ps ¼
FsðEÞ ꢀ
j
sðEÞ ꢀ dE
(2)
The relative quantum efficiency (h) of europium complexes C1–
C5 is estimated by assigning Rhodamine 6G as a reference
compound. The relative quantum efficiency of europium complexes
is calculated by MATLAB software having Rhodamine 6G compound
as reference and assuming their quantum efficiency value as 100.
The relative quantum efficiency of complexes C1–C5 is in the range
146.62–457.68 which is calculated according to the relation [32]
where Fs (E) corresponds to the shape of emission band of triplet
state of ligand (energy donor) and s (E) is shape of absorption
j
spectrum of metal ion (energy acceptor), 2p
Z2/h term is constant.
Table 4
The excited state energy of DPBD, bipy and phen ligands.
I
Astd n2
Ligands
Excited state
Excitation energy
cmꢁ1
h
¼
hstd
ꢀ
ꢀ
ꢀ
(1)
n2std
Istd
A
eV
DPBD
Bipy
Singlet (S1)
Triplet (T1)
Singlet (S1)
Triplet (T1)
Singlet (S1)
Triplet (T1)
32,154
25,125
29,900
22,900
31,000
22,100
3.98
3.11
3.71
2.83
3.84
2.74
h
is relative quantum efficiencies of the complex, hstd (0.5) is
quantum efficiencies of the reference compound, A and Astd (0.029)
are the absorbance’s at the excitation wavelength of the complex
and the reference compound, I and Istd are the integrated
intensities of corrected emission spectra of the complex and the
Phen

M. Bala et al. / Journal of Fluorine Chemistry 178 (2015) 6–13
11
The second is the thermal de-excitation mechanism dealing
4. Experimental
with the inverse transfer of energy from metal to ligand which is
represented thermal de-excitation constant K(T) [37] as shown
below
4.1. Starting materials and instrumentation
All starting materials were purchased from commercial
source of analytical grade and used without further purification.
The lanthanide nitrates (Eu(NO3)3ꢀ5H2O and Gd(NO3)3ꢀ5H2O)
were acquired from Sigma–Aldrich. The synthesized DPBD
ligand was recrystalized three timed with ethanol before the
complexation.
D
E=RTÞ
KðTÞ ¼ A ꢀ eðꢁ
(3)
where
DE is the energy difference between ligand lowest excited
triplet level and resonance energy level of metal ion. Therefore,
efficient energy transfer decided by the appropriate value of Ps
and K(T).
The europium content was ascertained by complexometric
titration with EDTA (ethylenediaminetetraacetate). Carbon,
hydrogen and nitrogen were executed by Perkin Elmer 2400
The intersystem crossing (ISC) is effective only when energy
CHN Elemental Analyzer. Infrared spectra (4000–400 cmꢁ1
)
gap
D
E (S1 ꢁ T1) is appropriated, therefore the
DE values for DPBD,
phen and bipy in ISC process is 7029 cmꢁ1, 8900 cmꢁ1, 7000 cmꢁ1
respectively which indicated the efficiency of intersystem
crossing in corresponding complexes C3 and C5. An empirical
were performed with KBr pellets on Perkin Elmer Spectrum
400 spectrometer. 1H NMR spectra were recorded on Bruker
Avance II 400 NMR spectrometer in CDCl3 solution with TMS as
internal standard. UV–vis absorption spectra were measured on
Shimadzu-2450 UV-vis spectrophotometer. Thermogravimetric
analyses were carried out by using SDT Q600 up to 1100 8C with
a heating rate of 20 8C/min under nitrogen atmosphere. Powder
X-ray diffraction (XRD) pattern were determined by Rikagu
rule of Latva et al’s implies that
5000 cmꢁ1 range for an optimal energy transfer from ligand
lowest triplet level to metal resonance level [38]. The values of
(T1-M3+ are found to be approx 7625 cmꢁ1 5400 cmꢁ1
D
E (T1-M3+) should be in 2000–
D
E
)
,
,
4600 cmꢁ1 for DPBD, bipy and phen respectively. The energy
gap between DPBD ligand and Eu3+ is not desirable for efficient
energy transfer so the absorbed energy of the DPBD ligand may be
transmitted to auxiliary ligands first and then finally to the Eu3+
ion ensuing higher luminescence intensity of ternary complexes
C2–C5 as compared to binary complex C1. At the same time,
inverse energy transfer process also determines the effectiveness
of entire energy transfer mechanism. The slow inverse energy
transfer process constitutes an efficient energy transfer mecha-
nism in complex. It is observed that the energy gap between
europium ion and bipy is more than that between europium ion
and phen which diminished the rate of inverse energy transfer
process more in bipy complex leading to higher luminescence
intensity as compared to phen complex. The above analyses
indicate that auxiliary ligands facilitate the sensitization process
which is also noticed in photoluminescence study.
Ultima IV diffractometer with CuK
a radiation at 40 kV tube
voltage and 40 mA tube current. Fluorescence and phosphores-
cence measurements were made on Hitachi F-7000 fluorescence
spectrophotometer equipped with a xenon lamp as the excita-
tion source. The decay time values of the complexes were
calculated by software of the spectrophotometer (FL solution for
F-7000).
4.2. Synthesis of ligand
The ligand DPBD was synthesized and their synthetic route is
illustrated in Scheme 1. To a solution of acetophenone (0.21 mL,
1.80 mmol) and dry THF (60 mL) added sodium hydride (0.70 g,
2.90 mmol), the resulting mixture was stirred at room tempera-
ture for 15 min. Then added methyl-difluoroacetate (0.59 g,
5.40 mmol) and stirred the solution for 12 h at room temperature.
The reaction mixture was evaporated till the solid residue was
obtained. The residue was dissolved in dichloromethane (15 mL)
then acidified with hydrochloric acid (0.5 M) and washed with
water [19]. The solid residue was recrystalized from ethanol. The
DPBD was obtained as dark brown solid in 62% yield with 47–51 8C
melting point. IR (KBr): cmꢁ1 3430 (b), 3080 (m), 2998 (m), 2890
(w), 1635 (s), 1542 (s), 1480 (s), 1360 (s), 1270 (s), 785 (s), 705 (s);
3. Conclusion
In summary, a series of five novel europium(III) C1–C5
complexes have been synthesized and characterized by elemen-
tal analysis, 1H NMR, IR, UV–vis absorption, powder XRD, TG/
DTA-DSC and photoluminescence (PL) spectroscopy. The differ-
ence between ligand and complexes IR spectra show that the
binding of ligand DPBD to the metal ion through oxygen atoms of
enol form of ligand. The characteristics peaks of europium (III) ion
in emission spectra and luminescence decay curves of the
complexes reveal that Eu3+ ion is located in a polarizable
chemical environment which is acting as only one luminescent
center. All photoluminescent features of the complexes reveal
that the europium ion is sensitized efficiently by the primary
ligand and auxiliary ligands as shown in proposed energy transfer
mechanism. These complexes possess significant impacts on
photophysical properties by the introduction of the auxiliary
ligands. These results demonstrate that the luminescence
properties enhanced effectively by the addition of auxiliary
1H NMR (400 MHz, CDCl3):
d 15.01 (s, 1H, enolic OH), 7.45–7.22
(m, 5H, Ar–H), 6.65 (t, 1H, CHF2), 6.35 (s, 1H, enol CH), 4.22 (s, 2H,
keto-CH2) ppm.
4.3. Synthesis of complexes
The synthesis of europium (III) complexes was accomplished by
a general procedure (Scheme 1): An alcoholic solution of DPBD
(0.63 g, 3.2 mmol) was added to the aqueous solution of (0.42 g,
1.0 mmol) europium nitrate pentahydrate with constant stirring
on magnetic stirrer. Then the resulting mixture was treated with
0.05 M aqueous NaOH to adjust the pH 6.5–7.0. After stirring for
4 h at a temperature of 50 8C, a white solid was filtered, purified by
washing with distil water and then with ethanol to remove the free
ligand. The solid was dried at 40 8C to obtain the powder
Eu(DPBD)3ꢀH2O (C1) complex.
ligands as the
p-conjugation system, stability of the complexes
increases and the efficient transfer of energy from ligand to metal
occurs. The series of crystalline europium complexes C1–C5
exhibiting excellent photoluminescent properties such as high
luminescence intensity with longer life time as well as good CIE
chromaticity coordinates and high thermal stability are promis-
ing red-emitting component for OLEDs having potential applica-
tion in display devices.
Eu(DPBD)3ꢀH2O (C1): white solid, yield 72%; IR (KBr): cmꢁ1
3421 (b), 3068 (m), 2983 (m), 2879 (w), 1626 (s), 1560 (s), 1529 (s),
1467 (s), 1352 (s), 1269 (s), 1109 (s), 767 (s), 704 (s), 530 (s), 428
(m); 1H NMR (400 MHz, CDCl3):
CHF2), 6.35 (m, 9H, Ar–H), 3.20 (s, 3H, enol CH). Anal. Calcd for
d 6.90 (m, 6H, Ar–H), 6.50 (t, 3H,

12
M. Bala et al. / Journal of Fluorine Chemistry 178 (2015) 6–13
Scheme 1. The synthetic route of ligand DPBD and its europium (III) complexes.

M. Bala et al. / Journal of Fluorine Chemistry 178 (2015) 6–13
13
EuC30H25O8F6: C, 46.21; H, 3.20; Eu, 19.51; found: C, 45.94; H, 2.92;
Eu, 19.02.
Similarly, the complexes C2–C5 were prepared with the same
Acknowledgement
One of the authors Manju Bala is grateful to the financial
support from Council of Science and Industrial Research (CSIR) of
India (09/382(0155)/2012-EMR-I).
process as adopted in the synthesis of complex C1, but the mixture
of DPBD (0.63 g, 3.2 mmol), biq (0.25 g, 1.0 mmol) and europium
nitrate (0.42 g, 1.0 mmol) was used for complex C2, the mixture of
DPBD (0.63 g, 3.2 mmol), phen (0.18 g, 1.0 mmol) and europium
nitrate (0.42 g, 1.0 mmol) for complex C3, the mixture of DPBD
(0.63 g, 3.2 mmol), neo (0.20 g, 1.0 mmol) and europium nitrate
(0.42 g, 1.0 mmol) for complex C4 and the mixture of DPBD (0.63 g,
3.2 mmol), bipy (0.15 g, 1.0 mmol) and europium nitrate (0.42 g,
1.0 mmol) for complex C5 were used to synthesized the
europium(III) ternary complexes.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.jfluchem.2015.
06.011.
Eu(DPBD)3ꢀbiq (C2): white solid, yield 70%; IR (KBr): cmꢁ1
3062 (m), 2978 (m), 2880 (w), 1622 (s), 1562 (s), 1525 (s), 1452
(s), 1352 (s), 1269 (s), 1109 (s), 765 (s), 700 (s), 522 (s), 428 (m);
References
[1] S.I. Weissman, J. Chem. Phys. 10 (1942) 214–217.
[2] J. Kido, K. Nagai, Y. Okamotto, T. Skotheim, Chem. Lett. 20 (1991) 1267–1270.
[3] C. Yang, J. Xu, J. Ma, D. Zhu, Y. Zhang, L. Liang, M. Lu, Polym. Chem. 3 (2012)
2640–2648.
[4] Y. Hasegawa, Y. Wada, S. Yanagida, J. Photochem. Photobiol. C 5 (2004)
183–202.
1H NMR (400 MHz, CDCl3):
d 7.90 (m, 6H, Ar–H), 7.20 (m, 6H, Ar–
H), 6.82 (m, 6H, Ar–H), 6.44 (t, 3H, CHF2), 6.30 (m, 9H, Ar–H), 3.08
(s, 3H, enol CH). Anal. Calcd for EuC48H33N2O6F6: C, 57.65; H,
3.30; N, 2.80; Eu, 15.21; found: C, 57.46; H, 2.98; N, 2.47; Eu,
14.90.
[5] V. Tsaryuk, V. Zolin, K. Zhuravlev, V. Kudryashova, J. legendziewicz, R. Szostak,
J. Alloys Compd. 451 (2008) 153–157.
Eu(DPBD)3ꢀphen (C3): white solid, yield 74%; IR (KBr): cmꢁ1
3057 (m), 2887 (w), 1625 (s), 1560 (s), 1531 (s), 1467 (s), 1350
(s), 1271 (s), 1111 (s), 769 (s), 704 (s), 530 (m), 429 (w); 1H
[6] K. Kuriki, Y. Koike, Y. Okamoto, Chem. Rev. 102 (2002) 2347–2356.
[7] M. Uekawa, Y. Miyamoto, H. Ikeda, K. Kaifu, T. nakaya, Synth. Met. 91 (1997)
259–262.
[8] S. Li, W. Zhu, Z. Xu, J. Pan, H. Tian, Tetrahedron 62 (2006) 5035–5048.
[9] S. Lis, Z. Piskula, K. Staninski, S. tamaki, M. Inoue, Y. Hasegawa, J. Rare Earths
26 (2008) 185–191.
[10] R.E. Whan, G.A. Crosby, J. Mol. Spectrosc. 8 (1962) 315–327.
[11] A. Sinha, in: C. Rao, J. Ferraro (Eds.), Spectroscopy in Inorganic Chemistry, vol.
2, Academic Press, New York, London, 1971.
[12] S. Sato, M. Wada, Bull. Chem. Soc. Jpn. 43 (1970) 1955–1962.
[13] M.T. Berry, P.S. May, H. Xu, J. Phys. Chem. A 100 (1996) 9216–9222.
[14] G.A. Hebbink, L. Grave, L.A. Woldering, D.N. Reinhoudt, F.C.J.M. Van Veggel, J.
Phys. Chem. A 107 (2003) 2483–2491.
NMR (400 MHz, CDCl3):
d 8.78 (s, 2H, Ar–H), 7.70 (m, 6H, Ar–
H), 6.88 (m, 6H, Ar–H), 6.48 (t, 3H, CHF2), 6.32 (m, 9H, Ar–H),
2.98 (s, 3H, enol CH). Anal. Calcd for EuC42H29N2O6F6: C, 54.60;
H, 3.14; N, 3.03; Eu, 16.46; found: C, 54.40; H, 3.04; N, 2.90; Eu,
16.12.
Eu(DPBD)3ꢀneo (C4): white solid, yield 70%; IR (KBr): cmꢁ1
3051 (s), 2989 (m), 2723 (w), 1608 (s), 1571 (s), 1502 (s), 1473 (s),
[15] N.S. Baek, M.K. Nah, Y.H. Kim, H.K. Kim, J. Lumin. 127 (2007) 707–712.
[16] L.M. Fu, X.C. Ai, M.Y. Li, X.F. Wen, R. Hao, Y.S. Wu, Y. Wang, J.P. Zhang, J. Phys.
Chem. A 114 (2010) 4494–4500.
1355 (s), 1269 (s), 1105 (s), 763 (s), 700 (s), 536 (s), 428 (m); 1
H
NMR (400 MHz, CDCl3):
d 7.88 (s, 2H, Ar–H), 7.46 (m, 4H, Ar–H),
[17] E. Deiters, F. Gumy, J.C.G. Bu¨nzil, Eur. J. Inorg. Chem. (2010) 2723–2734.
[18] J. Yu, R. Deng, L. Sun, Z. Li, H. Zhang, J. Lumin. 131 (2011) 328–335.
[19] M. Ra¨sa¨nen, H. Takalo, J. Rosenberg, J. Ma¨kela¨, J. Lumin. 146 (2014)
211–217.
[20] D. Wang, Y. Pi, C. Zheng, L. Fan, Y. Hu, X. Wei, J. Alloys Compd. 574 (2013)
54–58.
[21] D. Wang, C. Zheng, L. Fan, J. Zheng, X. Wei, J. Synth. Met. 162 (2012) 2063–
2068.
[22] G. Shao, Y. Li, K. Feng, F. Gan, M. Gong, Sens. Actuators B 173 (2012) 692–697.
[23] M.H.V. Werts, R.T.F. Jukes, J.W. Verhoeven, Phys. Chem. Chem. Phys. 4 (2002)
1542–1548.
6.85 (m, 6H, Ar–H), 6.45 (t, 3H, CHF2), 6.28 (m, 9H, Ar–H), 2.92 (s,
3H, enol CH), 2.45 (s, 6H, CH3). Anal. Calcd for EuC44H33N2O6F6: C,
55.52; H, 3.47; N, 2.94; Eu, 15.98; found: C, 55.27; H, 3.32; N, 2.78;
Eu, 15.85.
Eu(DPBD)3ꢀbipy (C5): white solid, yield 70%; IR (KBr): cmꢁ1
3064 (m), 2968 (w), 1620 (s), 1565 (s), 1517 (s), 1471 (s), 1352
(s), 1270 (s), 1105 (s), 767 (s), 704 (s), 532 (s), 428 (m); 1H NMR
(400 MHz, CDCl3):
d 8.10 (m, 4H, Ar–H), 7.40 (m, 4H, Ar–H),
6.80 (m, 6H, Ar–H), 6.50 (t, 3H, CHF2), 6.36 (m, 9H, Ar–H), 3.00
(s, 3H, enol CH). Anal. Calcd for EuC40H29N2O6F6: C, 53.39; H,
3.22; N, 3.11; Eu, 16.90; found: C, 53.21; H, 3.06; N, 2.98; Eu,
16.78.
In order to calculate the lowest triplet state of ligand DPBD, a
gadolinium (III) binary complex Gd(DPBD)3ꢀ(H2O)2 was also
prepared in similar way, as adopted in the synthesis of europium
complexes using the mixture of DPBD (0.63 g, 3.2 mmol) in
ethanol and aqueous solution of gadolinium nitrate (0.45 g,
1.0 mmol).
[24] B.R. Judd, Phys. Rev. 127 (1962) 750–761.
[25] G.S. Oflet, J. Chem. Phys. 37 (1962) 511–519.
[26] V. Divya, S. Biju, R.L. Varma, M.L.P. Reddy, J. Mater. Chem. 20 (2010) 5220–
5227.
[27] H.F. Li, P.F. Yan, P. Chen, Y. Wang, H. Xu, G.M. Li, Dalton Trans. 41 (2012) 900–907.
[28] B. Francis, D.B.A. Raj, M.L.P. Reddy, Dalton Trans. 39 (2010) 8084–8092.
[29] M. Bala, S. Kumar, P. Boora, V.B. Taxak, A. Khatkar, S.P. Khatkar, J. Mater. Sci.
Mater. Electron. 25 (2014) 2850–2856.
[30] V. Bekiari, P. Lianos, Adv. Mater. 10 (1998) 1455–1458.
[31] H.F. Brito, O.L. Malta, L.R. Souza, J.F.S. Menezes, C.A.A. Carvalho, J. Non-cryst.
Solids 247 (1999) 129–133.
[32] A. Brandner, T. Kitahara, N. Beare, C. Lin, M.T. Berry, P.S. May, Inorg. Chem. 50
(2011) 6509–6520.
[33] P. Gawryszewska, J. Sokolnicki, J. Legendziewicz, Coord. Chem. Rev. 249 (2005)
2489–2509.
[34] N.C. Greenham, I.D.W. Samuel, G.R. Hayes, R.T. Philips, Y.A.R.R. Kessener, S.C.
Moratti, A.B. Holmes, R.H. Friend, Chem. Phys. Lett. 241 (1995) 89–96.
[35] X. Yu, Q. Su, J. Photochem. Photobiol. A: Chem. 155 (2003) 73–78.
[36] D.L. Dexter, Chem. Phys. 21 (1953) 836–850.
[37] C.R.S. Dean, T.M. Shephred, J. Chem. Soc. Faraday Trans. 2 71 (1975) 146–155.
[38] M. Latva, H. Takalo, V.M. Mukkala, C. Matachescu, J.C. Rodriguez Ubis, J.
Kankare, J. Lumin. 75 (1997) 149–169.
Gd(DPBD)3ꢀ(H2O)2 (C6): white solid, yield 62%; IR (KBr): cmꢁ1
3429 (b), 3070 (m), 2985 (m), 2875 (w), 1618 (s), 1564 (s), 1528 (s),
1468 (s), 1352 (s), 1269 (s), 1107 (s), 768 (s), 702 (s), 530 (s), 428
(m); 1H NMR (400 MHz, CDCl3):
d 6.87 (m, 6H, Ar–H), 6.47 (t, 3H,
CHF2), 6.38 (m, 9H, Ar–H), 3.03 (s, 3H, enol CH). Anal. Calcd for
GdC30H25O8F6: C, 45.91; H, 3.18; Gd, 20.0; found: C, 45.84; H, 3.01;
Gd, 19.62.